1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor for use in copiers, facsimile machines, laser printers, and direct digital platemakers. In addition, the present invention also relates to a method of manufacturing electrophotographic photoreceptor, and an image forming apparatus and a process cartridge using the electrophotographic photoreceptor.
2. Discussion of the Related Art
Recently, the mainstream of electrophotographic photoreceptor (hereinafter simply “photoreceptor”) has switched from inorganic photoreceptor that uses inorganic materials such as selenium, zinc oxide, and cadmium sulfide, to organic photoreceptor that uses organic materials. Organic photoreceptors are more advantages than inorganic photoreceptors in reduction of environmental load and manufacturing cost and flexibility in compositional designing. Nowadays, the production of organic photoreceptors comes close to 100% of the total production of photoreceptors. Recently, organic photoreceptors are sought to be mechanical components rather than consumable supplies in accordance with increasing momentum of global environmental protection.
Various attempts have been made to improve durability of organic photoreceptors. For example, Japanese Patent Application Publication No. (hereinafter “JP-A”) 2000-66424 and JP-A 2000-171990 have proposed forming a cross-linked resin layer and a zol-gel cured resin layer, respectively, on the surface of a photoreceptor. The former is more productive because the layer is likely to neither fracture nor crack even when an electron transport material is added thereto. In particular, radical-polymerized acrylic resins are advantages because of having high stiffness and providing the resultant photoreceptor with high photosensitivity. Because of including plural chemical bonds, such cross-linked or cured resin layers may not be immediately brought into abrasion even when a part of chemical bonds are cut by application of mechanical stress.
On the other hand, the mainstream of electrophotographic toner (hereinafter simply “toner”) is switching from irregular-shaped toner to spherical toner. Spherical toners have an advantage in production of high quality image.
Generally, spherical toners are manufactured by chemical manufacturing methods such as a suspension polymerization method, an emulsion aggregation polymerization method, an ester elongation method, and a dissolution suspension method. Spherical toners may be hereinafter referred to as “polymerization toners” as appropriate. For example, a typical polymerization toner for use in an electrophotographic image forming apparatus may have an average circularity of from 0.95 to 0.99 and shape factors SF-1 and SF-2 of from 110 to 140. A true sphere has a circularity of 1.0 and shape factors of 100.
Because of having a uniform shape, each polymerization toner particles has a uniform charge quantity. Therefore, polymerization toner particles develop a latent image into a toner image with high sharpness, resolution, and gradation. In addition to this high developability, polymerization toner particles also have high transfer efficiency. Also, because polymerization toner particles are easy to include wax, advantageously, oil is not needed when fixed on recording media such as paper.
At the same time, disadvantageously, polymerization toner particles are difficult to be removed when remaining on a photoreceptor. To solve this problem, a greater amount of external additives are required on the surfaces of the polymerization toner particles. As a result, the external additives may form undesired films thereof on the photoreceptor.
In order to more effectively remove residual polymerization toner particles from a photoreceptor, one proposed approach involves applying a solid lubricant (e.g., zinc stearate) to a surface of a photoreceptor to reduce the surface friction coefficient thereof, as described in a technical document entitled “Blade cleaning system for polymerized and small size toner, Hyakutake et al., Japan Hardcopy Fall Meeting, 2001, 24-27”.
However, a solid lubricant (e.g., zinc stearate) may not be sufficiently applied to a surface of a photoreceptor when the surface is relatively smooth.
In view of this, JP-A 2007-79244 proposed to roughen a surface of a photoreceptor so that a lubricant is reliably applied thereto, in other words, receptivity of the surface to a lubricant is improved. It is disclosed therein that a surface of a photoreceptor preferably has a surface roughness Rz (defined in JIS B0601-1994) of from 0.4 to 1.0 μm, which can be achieved by addition of a filler to the surface layer.
Although having the same Rz value, the surface profile of a photoreceptor may vary. For example, even when the average distance between the highest peak and lowest valley is extremely different, the Rz value may be the same. Therefore, receptivity of the surface to zinc stearate may not be improved only by controlling the Rz value.
In addition to Rz (ten points average roughness), Ra (arithmetical average roughness), Rmax (the maximum height), etc., all defined in JIS B0601, are also widely used for evaluation of surface roughness. However, these parameters are not satisfactory to evaluate surface roughness of photoreceptors, and various proposals have been made as follows.
JP-A 07-104497 discloses a method in which a divisional breadth X having an average line 2 at center is assumed on a cross section curve 1 obtained through the measurement of surface contour with a surface roughness measuring device, and the surface contour is evaluated on the basis of the number of peaks 4 of a pair of adjacent crest and trough outside the breadth X per unit length L. In this case, a substrate having the number of the peaks 4 equal to or less than 100 with the breadth X kept at 20 μm and the unit length L at 1 cm is used to fabricate an organic photoreceptor.
To form high quality image by solving the problem of insufficient cleaning of small-size toner, JP-A 2002-196645 discloses an image forming device in which a cleaning roll having bias voltage applied to separate electrified toner from a photoreceptor is provided on the upstream side of a cleaning blade, and a photoreceptor having a surface roughness Rz of 0.1 μm to 2.5 μm in an average of ten points is provided.
JP-A 2006-163302 discloses an electrophotographic photoreceptor which satisfies ΔT>Rz and 0 μm<ΔT+Rz<5 μm, wherein ΔT and Rz represent the wear amount and the surface roughness, respectively, per kilocycle of the electrophotographic photoreceptor.
JP-A 2007-86319 discloses an electrophotographic photoreceptor in which the surface of the photosensitive layer is subjected to surface roughening treatment, and when the glossiness of the surface of the photosensitive layer after the surface roughening treatment is measured, a standard deviation of the measurement value is ≦4.
Japanese Patent No. 3040540 (corresponding to JP-A 04-243265) discloses an image forming system which includes a blade, an image forming member, and a toner composition. The image forming member has a surface on which the toner composition forms an image, and the surface has surface roughnesses defined by the following inequations:R/ann4>KB(1−σ2)/32πEt2af R/ann2>√{square root over (3)}/8π2·(1+μ2)/μ·KB/Γ·t/af·θwherein R is the average height of the projecting parts of the surface; ann is half the nearest adjacent distance between the projecting parts on the surface; KB is the bulk elastic modulus of the blade; σ in the Poisson's ratio of the toner composition; E is the Young's modulus of the toner composition; t is the average thickness of flat particles in the toner composition; of is the average radius of the flat particles; μ is the average between the coefficient of friction of the toner blade and the coefficient of friction of toner surfaces; Γ is the Dupre' work of the adhesion between the surface and the flat particles; and θ is a blade tip angle.
Japanese Patent No. 3938209 (corresponding to WO 05/093518) discloses a cylindrical electrophotography photosensitive body having a cylindrical support and an organic photosensitive layer provided on the cylindrical support. The circumferential surface of the electrophotography photosensitive body has recesses of dimple shape. The ten point average height Rzjis (A) measured by scanning in the circumferential direction of the circumferential surface of the electrophotography photosensitive body is 0.3 to 2.5 μm, and the ten point average height Rzjis (B) measured by scanning in the direction of the generating line of the circumferential surface of the electrophotography photosensitive body is 0.3 to 2.5 μm. The average interval RSm (C) of the irregularities measured by scanning in the circumferential direction of the circumferential surface of the electrophotography body is 5 to 120 μm, and the average interval RSm (D) of the irregularities measured by scanning in the direction of the generating line of the circumferential surface of the electrophotography photosensitive body is 5 to 120 μm. The ratio (D/C) of the average interval RSm (D) of the irregularities to the average interval RSm (C) of the irregularities is 0.5 to 1.5.
Japanese Patent No. 3938210 (corresponding to WO 05/093520) discloses an electrophotographic photosensitive member having a support and an organic photosensitive layer provided on the support. A plurality of dimple-shaped concavities are formed on the surface of the surface layer of the electrophotographic photosensitive member and a plurality of recesses corresponding to the dimple-shaped concavities formed on the surface of the surface layer are formed on the interface between the surface layer and a layer directly under the surface layer.
JP-A 2005-345788 discloses an image forming apparatus having a plurality of image carriers which are each obtained by disposing a photosensitive layer on a conductive support and on which electrostatic latent images are formed by surface exposure, a plurality of development devices, and a plurality of cleaning means. At least a pair of development devices among a plurality of developing devices house developers having the same hue but different from each other in luminosity. The compositions or physical values of surface layers of the image carriers are set according to the luminosity of the developers housed in the plurality of development devices.
JP-A 2004-258588 discloses an image forming apparatus including a photoreceptor having a surface performance that the ten-point average roughness RzJIS is ≧0.1 μm and ≦1.5 μm or the maximum height Rz is 2.5 μm or less, and the frictional resistance Rf is ≧45 gf and ≦200 gf.
JP-A 2004-054001 discloses an image forming method including a primary transferring process transferring a toner image visualized by developing onto an intermediate transfer body, a secondary transferring process transferring the toner image transferred onto the intermediate transfer body onto a recording medium, and a cleaning process in which residue toner on an electrophotographic photoreceptor is eliminated after transferring the toner image onto the recording material. The surface roughness Ra of the electrophotographic photoreceptor is 0.02 to 0.1 μm, the surface roughness Rz of the intermediate transfer body is 0.4 μm to 2.0 μm, and image formation is carried out by feeding surface energy lowering agent to the surface of the electrophotographic photoreceptor.
JP-A 2003-270840 discloses an image forming apparatus including an organic photoreceptor in which the average cycle of the surface irregularity is 10 times or more the volume average particle diameter of the toner used therefor.
JP-A 2003-241408 discloses an electrophotographic device including an electrophotographic photoreceptor rotating at a peripheral speed of 200 mm/sec or more and a cleaning means. The electrophotographic photoreceptor has a photosensitive layer and a surface protection layer on a conductive base. The surface protection layer contains 35.0 to 45.5 mass % fluorine atom containing resin particles for the total mass of the protection layer and is of 0.1 to 5.0 μm in 10-point mean surface roughness, 0.1 to 10.0 in surface hardness by a Taber's abrasion resistant test method, and 0.1 to 0.7 in surface friction coefficient. The cleaning means is a rubber elastic body blade and the linear pressure of the blade to the electrophotographic photoreceptor is 0.294 to 0.441 N/cm. The glass transition point (Tg) of toner in use is 40 to 55° C. The tensile modulus (Young's modulus) as a blade property value is 784 to 980 N/cm2. The value of repulsive elasticity is 35 to 55%. Fluorine atom resin particulates are incorporated in the surface of the base material.
JP-A 2003-131537 discloses an image forming method including and image forming body on which a toner forms an image, which satisfies the equation d/t×0.01≦Ra≦0.5, wherein d/t represents the degree of flatness of the toner particle (d represents the volume average particle diameter of the toner and t represents the thickness t of the toner particle) and Ra (μm) represents a surface roughness of the image forming body.
JP-A 2002-296994 and JP-A 2002-258705 each disclose an image forming apparatus including an image bearing member on which a spherical toner forms an image, the surface of which has convexities and concavities which are smaller than the volume average particle of the toner.
JP-A 2002-082468 discloses an electrophotographic device having an electrophotographic photoreceptor which is rotated above 200 mm/sec in circumferential speed and a cleaning means. The photoreceptor has a photosensitive layer and a surface protective layer on a conductive substrate. The surface protective layer contains 15.0 to 40.0 mass % fluorine atom-containing resin particles. Its surface roughness is 0.1 to 5.0 μm in ten-point mean roughness, its surface hardness is 0.1 to 20.0 in a taper wear test method, and its surface friction coefficient is 0.001 to 1.2.
JP-As 2001-265014, 2001-289630, 2002-251029, 2002-296822, 2002-296823, 2002-296824, 2002-341572, 2006-53576, 2006-53577, and 2006-79102 disclose methods of evaluating surface profile using Fourier transform. In Fourier transform, a variation which occurs with high frequency in a signal can be transformed into a frequency distribution, however, a variation which occurs with low frequency cannot. Further, disadvantageously, it is not clearly determined from the results of Fourier transform where the variation occurred.
JP-A 2004-117454 discloses a method for evaluating the surface roughness of a base body for an electrophotographic photoreceptor. A cross section curve stipulated by JIS B0601 from an arbitrary position of the base body surface is obtained in the abscissa direction by a length of 100 μm, positions of the cross section curve in the ordinate direction at equispaced positions in the abscissa direction are measured, variance of the positions stipulated by JIS Z8101 is obtained, one or more measurements selected from surface roughnesses Ra, Rz and Ry stipulated by JIS B0601 are also obtained, and the surface roughness of the base body is evaluated using the variance and the one or more measurements.
JP-A 2004-61359 discloses a surface roughness evaluating method of a component for an image forming device. A cross-section curve defined by JIS B0601 is found on the surface condition of the component to perform multiple resolution analysis on positional data rows in a surface roughness direction at equally spaced positions on the cross-section curve, and the state of the surface roughness is evaluated at least based on the result.
JP-A 2007-292772 discloses a surface roughness evaluating method of a component for an image forming device. A profile curve defined in JIS B0601 is calculated on a surface condition of the image forming apparatus component such as the electrophotographic photoreceptor substratum. Multiple resolution analysis such as wavelet transformation of a positional data row in the surface roughness direction at equal intervals on the profile curve is performed, and the surface roughness condition is evaluated based on the results.
JP-A 2008-70540 discloses a latent electrostatic image bearing member having a cross-linked surface layer which comprises a reactive silicone material. It is disclosed therein that such a surface layer is smooth and has a low surface energy, providing good cleanability of toners.
However, none of the above-described methods of evaluating surface roughness is sufficient to evaluate cleanability (removability) of small-size toners and polymerization (spherical) toners from photoreceptor.
In view of this situation, surface roughness may be evaluated from a surface profile recorded on a chart by a surface roughness & profile shape measuring instrument. However, it requires skill to evaluate surface roughness from the chart.
Namely, any of the parameters (Ra, Rmax, Rz, etc.) does not sufficiently represent cleanability (removability) of small-size toners and polymerization (spherical) toners from photoreceptor.
In the above-cited reference JP-A 2007-79244, a photoreceptor including alumina particles as a filler is disclosed. Since alumina particles are difficult to be uniformly dispersed in a coating material, it requires a special technique to form a uniform coating layer. A photoreceptor including polymethylsilsesquioxane particles is also disclosed therein. Such a photoreceptor does not always sufficiently hold a solid lubricant hereon because of having too large convexities and concavities on the surface thereof.
Although a coating material for forming a cross-linked resin surface layer has a low viscosity because of being comprised primarily of monomer components, silicon-containing particles such as silica particles and silicone resin particles can be stably dispersed therein. Accordingly, silicon-containing particles have a great advantage over various filler materials in manufacturing photoreceptors. However, silicon-containing particles do not always provide the resultant photoreceptor with high receptivity to solid lubricants, as seen in Example 2 of JP-A 2005-99688. This is because the surface of the photoreceptor has too large convexities and concavities to hold solid lubricants.
JP-A 08-248663 discloses a photoreceptor in which a photosensitive layer is provided on a conductive base. Surface roughness of the conductive base is 0.01-2.0 μm while surface roughness of the outermost surface side layer is 0.1-0.5 μm. And in the outermost surface side layer, inorganic grains with an average grain diameter of 0.05-0.5 μm are contained.
As described therein, the inorganic grain such as silica particle may be hydrophobized in an attempt to improve durability and prevent deterioration of image resolution which occurs by adhesion of contaminants such as corona products. However, the hydrophobized inorganic grain may not prevent adhesion of corona products while preventing adhesion of water, thereby causing image blurring.
In view of this situation, JP-A 2004-138643 proposed to use alumina as a filler to prevent image blurring. However, a special technique is required to disperse alumina in a cross-linked resin surface layer, as described above.
As seen above, the receptivity of a photoreceptor to solid lubricants may affect abrasion rate of the photoreceptor and/or cleanability (removability) of toner, i.e., printing quality. However, any technique to improve the receptivity of a highly-durable photoreceptor having a cross-linked resin surface layer has not been proposed so far.
Advantageously, such a photoreceptor having a surface layer such as a cross-linked resin layer has extremely high durability. On the other hand, disadvantageously, such a surface layer has a low receptivity to solid lubricants and therefore cleanability (removability) of polymerization toners is poor. As a result, highly-durable photoreceptors having a surface layer cannot be practically used in combination with polymerization toners under the current situation.